In recent years, noises arising from transformers for electric power have become environmentally problematic. A primary cause of the noise of a transformer for electric power is the magnetostriction of a grain-oriented electrical steel sheet used in the core of the transformer. To reduce the transformer noise, the magnetostriction of the grain-oriented electrical steel sheet needs to be reduced. An industrially advantageous solution is to coat the grain-oriented electrical steel sheet with an insulation coating.
Properties required for insulation coatings for grain-oriented electrical steel sheets include tension induced by a coating, moisture-absorption resistance, rust resistance, and lamination factor. Among these properties, it is important to secure tension induced by a coating for the purpose of the reduction of magnetostriction. The term “tension induced by a coating” as used herein means tension imparted to a grain-oriented electrical steel sheet by the formation of an insulation coating.
A coating on a grain-oriented electrical steel sheet includes a ceramic forsterite sub-coating formed by secondary recrystallization annealing and a phosphate-based insulation sub-coating disposed thereon. Known techniques for forming such an insulation coating are those disclosed in Japanese Unexamined Patent Application Publication No. 48-39338 and Japanese Unexamined Patent Application Publication No. 50-79442. In these techniques, steel sheets are coated with treatment solutions for insulation coating each containing colloidal silica, a phosphate, and a chromium compound (for example, one or more selected from chromic anhydride, a chromate, and a bichromate) and then baked.
Insulation coatings formed by these techniques have an advantage that magnetostrictive properties thereof are improved by applying tensile stress to grain-oriented electrical steel sheets. These treatment solutions contain a chromium compound, such as chromic anhydride, a chromate, or a bichromate, serving as a component for maintaining the moisture-absorption resistance of the insulation coatings well and therefore contain hexavalent chromium derived from the chromium compound. Japanese Unexamined Patent Application Publication No. 50-79442 also discloses a technique using no chromium compound. However, such a technique is extremely disadvantageous in view of moisture-absorption resistance. Hexavalent chromium contained in the treatment solutions is reduced into trivalent chromium, which is harmless, by baking. However, there is a problem in that various costs are incurred in treating the waste treatment solutions.
Japanese Examined Patent Application Publication No. 57-9631 discloses a treatment solution for insulation coating. The treatment solution is a so-called “chromium-free” treatment solution, for insulation coating for grain-oriented electrical steel sheets, containing substantially no chromium and contains colloidal silica, aluminum phosphate, boric acid, and one or more selected from sulfates of Mg, Al, Fe, Co, Ni, and Zn. Japanese Examined Patent Application Publication No. 58-44744 discloses a treatment solution, for insulation coating, containing colloidal silica, magnesium phosphate, boric acid, and one or more selected from sulfates of Mg, Al, Mn, and Zn. The use of the treatment solutions disclosed in Japanese Examined Patent Application Publication Nos. 57-9631 and 58-44744 is problematic in recent requirements for coating properties such as tension induced by a coating and moisture-absorption resistance.
Japanese Patent No. 2791812 discloses colloidal solutions (a particle size of 80 to 3000 nm) of oxides, carbides, nitrides, sulfides, borides, hydroxides, silicates, carbonates, borates, sulfates, nitrates, or chlorides containing Fe, Ca, Ba, Zn, Al, Ni, Sn, Cu, Cr, Cd, Nd, Mn, Mo, Si, Ti, W, Bi, Sr, and/or V. The colloidal solutions are used as additives for treatment solutions, for insulation coating, containing colloidal silica and a phosphate. These additives are used to improve the slippage (sticking resistance (removal property of stiction)) of and lubricity of insulation coatings such that troubles are avoided during the working of sheets into cores. The treatment solutions disclosed in Japanese Patent No. 2791812 need to contain a chromium compound. Japanese Patent No. 2791812 discloses no specific solutions or countermeasures to the above problems due to the use of chromium.
It could therefore be helpful:                to prevent a reduction in tension induced by a coating and a reduction in moisture-absorption resistance which are issues involved in causing treatment solutions for insulation coating to be chromium-free;        to provide a chromium-free treatment solution for insulation coating for grain-oriented electrical steel sheets, the chromium-free treatment solution being useful in achieving tension induced by a coating, moisture-absorption resistance, rust resistance, and lamination factor which are substantially equal to those obtained by the use of a chromium-containing treatment solution for insulation coating and which are properties required for insulation coatings for grain-oriented electrical steel sheets; and        to provide a method for producing a grain-oriented electrical steel sheet having an insulation coating using the chromium-free treatment solution for insulation coating for grain-oriented electrical steel sheets.        